Floating offshore structures such as e.g. oil platforms, offshore wind turbines, and offshore meteorological towers are known.
Several configurations have been proposed for the floating or buoyancy of structures: many of these employ floater elements in the form of substantially hollow floater tanks that in use are arranged substantially below the sea level and provide a buoyancy force to support the structure, e.g. a wind turbine. Ballast and/or mooring lines anchored to the seabed are provided for achieving stability.
In some of these floating structures, the buoyancy structure is designed to provide an excess buoyancy force and is maintained floating under the sea level by taut mooring lines tensioned by the excess buoyancy force.
For example, concepts have been developed such as the “Taught Leg Buoy” (TLB) floating wind turbine, with a slender cylindrical buoy and two or more sets of tensioned mooring lines, inclined relative to the seabed and connected to gravity anchors and to the buoy; or such as the “Tension Leg Platform” (TLP) floating wind turbine, in which the tensioned mooring lines are substantially vertical and are connected between gravity anchors on the seabed and arms or braces extending radially outwards with respect to the vertical axis of the wind turbine. The arms of the TLP may be part of the buoyancy structure, for example in the form of hollow spokes that extend radially outwards from a hollow central hub, or may be arranged above the sea level, in which case the buoy may be a slender cylindrical tank like in the TLB concept.
In order for these structures to be stable, the mooring lines must always be under tension; otherwise the offshore structure could become unstable and could fall over. Since the loads on the offshore structures, and particularly on a floating wind turbine may vary considerably both in magnitude and direction, a high amount of excess buoyancy and high tension in the cables may be required.
The buoyancy structures of floating offshore structures, particularly of floating wind turbines, may be subject to several loads, such as for example the weight of the wind turbine itself, impacts, forces exerted by waves, currents and tides, and also aerodynamic forces associated with the wind, rotor rotation, etc. Furthermore, like any body submerged in water, buoyancy structures are subject to hydrostatic pressure on the outer walls that are in contact with the sea water, and consequent buckling.
Buoyancy structures are also subjected to great loads especially derived from buoyancy variations because buoyancy normally varies considerably with sea level changes, i.e. tide changes or waves. At low tide, the sea level becomes lower and the tension in the mooring lines can be relatively low (minimum required tension). In these cases care should be taken not to reach zero. When the sea level rises at e.g. high tide, the tension in the mooring lines can be significantly higher, i.e. in this example, the tension in the mooring lines at high tide would be equal to the minimum required tension (the tension at low tide) plus the entire buoyancy variation. If the buoyancy variation is reduced, the tension variation in the mooring lines is also reduced, but at the same time stability is maintained.
There thus exists a need for floating offshore structures that substantially reduce buoyancy variations, but at the same time maintain their stability.